Along with miniaturization of transistors, in a thin film such as an insulating film forming a sidewall spacer (SWS) or the like of a gate electrode, a decrease in a film forming temperature, a high resistance to hydrogen fluoride (HF), and a decrease in a dielectric constant are necessary. Accordingly, as the insulating film, employing a silicon oxycarbide film (SiOC film) obtained by adding oxygen (O) and carbon (C) to a silicon film (Si film), a silicon oxynitride film (SiON film) obtained by adding O and nitrogen (N) to the Si film, or a silicon oxycarbonitride film (SiOCN film) obtained by adding O, C and N to the Si film has been studied.